Cemented carbide cutting tools and inserts (generally “cutting tools”) are commonly employed in machining operations such as, for example, cutting, drilling, reaming, countersinking, counterboring, end milling, turning, grooving, threading, and tapping. The manufacturing process for cemented carbide cutting tools involves consolidating metallurgical powder (comprised of hard particles and a binder) to form a compact. The compact is then sintered to form a cylindrical tool blank having a solid monolithic construction. Subsequent to sintering, the tool blank may be appropriately machined to form a cutting edge or other features of the particular geometry desired on the cutting tool.
Cemented carbide tools are industrially important because of the combination of tensile strength, wear resistance, and toughness that is characteristic of these materials. The hard particles may be, for example, carbides, nitrides, borides, silicides, or oxides of elements within groups IVB through VIB of the periodic table. A common example is tungsten carbide. The binder may be a metal or metal alloy, typically cobalt, nickel, iron or alloys of these metals. The binder “cements” the hard particles within a continuous matrix interconnected in three dimensions.
The physical and chemical properties of cemented carbide materials depend in part on the individual components of the metallurgical powders used to produce the material. The properties of the cemented carbide materials are determined by, for example, the chemical composition of the hard particle, the average particle size and particle size distribution of the hard particles, the chemical composition of the binder, and the ratio of binder to hard particle in the substrate. By varying the components of the metallurgical powder, cutting tools, including cutting inserts, may be produced with unique properties matched to specific applications. Many cemented carbide cutting tools are prepared with in cobalt as the primary component of the binder, such cemented carbides may be particularly useful for machining today's mold and die materials. The weight percentage of cobalt as a binder in such cemented carbide cutting tools, typically, ranges from 5 to 20%.
Presently, a very limited amount of cemented carbide cutting tools have been prepared with ruthenium added to a cobalt binder. According to the reference book Modern Metal Cutting by Sandvik (ISBN-9197229930, 1996), typical substrates of cutting inserts are tungsten-based carbide (WC), cubic boron nitride (CBN), ceramic (AI203/Si3N4), titanium-based carbide or cermet (TiC/TiN), coronite (combined high speed steel and carbide) and polycrystalline diamond (PCD). According to the reference book World Directory and Handbook of Hardmetals and Hard Materials, 5th Edition (ISBN-0950899526), 1992, by K. J. Brookes, which collects carbide data from all major carbide cutting tool manufacturers worldwide, almost all tungsten carbides use cobalt as a binder with addition of a balance of some other alloying compounds, such as TiC, TaC/NbC, to refine the properties of the substrate for particular applications.
Ruthenium (Ru) is a member of the platinum group and is a hard, lustrous, white metal that has a melting point of approximately 2,500° C. Ru does not tarnish at room temperatures, and may be used as an effective hardener, creating alloys that are extremely wear resistant. Stellram, an Allegheny Technologies Company located at 1 Teledyne Place, LaVergne, Tenn., USA 37086, has found that adding an amount of Ru into the cobalt binder continuous phase of a tungsten carbide substrate results in a cemented carbide cutting insert with improved resistance to thermal cracking and significant reduction of the propagation of cracks along and beyond the cutting insert edges and propagation of cracks into the substrate during use in machining processes. Such substrates may be called Ru-featured cemented carbides. The amount of Ru may be varied depending on the application, however, typical commercially available products include a concentration of ruthenium in the binder phase of cemented carbide substrates in the ranges of approximately 5% to 25%, by weight.
The cemented carbide substrates may additionally include a single or multiple layer coating to enhance cutting performance of tungsten carbide cutting inserts. Methods for coating cemented carbide cutting tools include chemical vapor deposition (CVD), physical vapor deposition (PVD) and diamond coating. Most often, CVD is used to apply the coating to cutting inserts due to the well-known advantages of CVD coatings in cutting tools. It is well known that PVD coatings are thinner than CVD coatings, thus provide the advantage of retaining sharp cutting edges.
As an example of PVD coating technologies, U.S. Pat. No. 6,352,627 B2 discloses a PVD coating method and device, which is based on magnetron sputtering techniques to produce refractory thin films or coats on cutting inserts, can deliver three consecutive voltage supplies during the coating operation, promoting an optimally enhanced ionization process that results in good coating adhesion on the substrate, even if the substrate surface provided is rough, for example because of grinding or jet abrasion treatment. Examples of some other PVD coating technologies are ion plating, arc discharge evaporation, ion beam assisted deposition and activated reactive evaporation.
Diamond compacts, or composite diamond compacts, may contain Ru as a significant element in the substrate material (no more than 20% by volume), such as in U.S. Pat. No. 6,620,375 B1, European Patent 1,077,783. Diamond compacts may also be called polycrystalline diamond (PCD) and may be manufactured under elevated temperature and pressure conditions. Diamond compacts are brittle in nature and thus they have to be bonded to a substrate that contains a binder, such as cobalt, iron or nickel, that may further include Ru. A diamond compact contains a polycrystalline mass of diamond particles presented in an amount of at least 80% by volume of the substrate. Diamond compacts are typically used in abrading or abrasive tools for sawing, milling or profile cutting of wood products.
Ru-featured cemented carbide cutting inserts are limited to either uncoated or CVD coated, that is to say, no PVD coatings have not been applied to Ru-featured carbide cutting inserts. For example, X500™, a commercial designation of Stellram's cutting tool products, is a multiple layer TiN—TiC—TiN CVD coated carbide cutting insert having Ru-featured substrate for the applications in machining of titanium alloys, nickel based alloys and ductile iron; and X44™ and X22™ (both commercial designations of Stellram's cutting tool products) are uncoated Ru-featured cemented carbide cutting inserts for applications in machining of steels and alloyed steels.
Different from conventional cobalt-based binder phase, in a Ru-featured cemented carbide substrate, cobalt (Co) and ruthenium (Ru) act as a mixed solvent during the sintering process. It is known that a cemented carbide cutting inserts with cobalt as a binder have a tendency for cobalt to penetrate through the surface of the compact and melt during the sintering process forming cobalt structures on the surface. This process is often referred to as cobalt capping. Cobalt caps on the substrate surface are randomly distributed, thus creating a crested and rough texture on the surface of the coating tool. The presence of Ru in the cobalt binder exaggerates the cobalt capping on cemented carbides, increasing the height and frequency of the cobalt caps. Even though some surface treatment techniques may be performed to reduce the cobalt capping effect to some degree, it is difficult to consistently produce a uniform surface on a sintered cemented carbide cutting inserts containing Ru in the binder. However, commercially available coated tools have compensated for the enhanced cobalt capping effects on the surface of the carbide cutting inserts including Ru-featured binders by applying a thick CVD coating with or without some pre-surface treatment methods like electropolishing, micro-blasting, wet blasting, compressed air blasting, etc. Thick CVD coating layers may cover up and reduce the overall impact of the cobalt capping. Additionally, the elevated CVD coating temperature (usually above 1500° C.) slightly melts the surface region of the cemented carbide and promotes better adhesion of the CVD coating layer to the surface. Thus, as of today, the Ru-featured carbide cutting inserts are limited only to uncoated and CVD coated inserts.
A PVD coating process involves a much lower coating temperature and, therefore, does not remelt the surface binder and tends to produce coatings that are not as well adhered to the carbide substrate surface. Additionally, thin PVD coatings cannot compensate for the enhanced cobalt capping effect. This is believed to be the reason that Ru-featured carbide cutting inserts are limited to uncoated or CVD coated cutting tools. The apparent difficulties of applying the low operation temperature based thin PVD coatings to guarantee consistent surface quality of coated carbide cutting inserts has not been considered feasible.